The prevention of back siphonage and back flow of water in a water supply system is a problem to which considerable attention need be given. This is particularly so where chemicals or gases are introduced in water coming from a sanitary water supply wherein the return of water from the end of the system where the chemical or gas is discharged could cause a contamination of the water supply. Such is the case in carbonated systems which utilize a carbonated gas in conjunction with a sanitary water supply system and in many steam cleaning operations which utilize chemicals on the discharge end. Prevention of backflow is also an important consideration in many other fluids supply systems other than water.
Conventional check valve arrangements have been utilized which permit a flow of fluid in one direction only. However, quite often the check valve member will fail resulting in a back flow of fluid through the fluid supply line. Therefore, many prior art check valve arrangements have been developed which utilize a safety pressure relief port to vent the backflow of fluid to the atmosphere should the check valve fail.
One such prior device is disclosed in U.S. Pat. No. 2,953,153 wherein a vented safety check valve is provided having a pair of spring biased pistons. The first piston is on the upstream side of a second master piston and functions to uncover a vent port to the atmosphere upon detecting failure of the master piston to seat properly and prevent backflow. Thus, should the master piston fail to seat on its seating rim, the first piston will be raised off of its valve seat by the back flow to vent the backflow to the atmosphere. Another much simpler device is shown in U.S. Pat. No. 3,346,001 wherein an atmospheric vent is provided for venting backflow to the atmosphere. A flexible valve member permits flow in one direction, however, when backflow occurs the flexible valve member is seated to prevent backflow through the valve while opening the vent to the atmosphere. While the prior devices have provided venting of backflow to the atmosphere upon failure of the main check valve member, it is desirable to provide a valve arrangement having increased reliability in its checking function in conjunction with more simplified safety venting.